ProCoach

If we can work out schedules so I send it to you for a weekend and get it back right away, you could try my VBOX Sport and see...

dbbarron

I have done further analysis and now realized a big mistake in my original methodology. I was using the video immediately viewable within HLT as my reference and assuming constant offset from start and stop triggers - this was a mistake.

I have now overlaid and exported 5 laps with good data from the afternoon.

First, I compared clock time to pass the first clear visual reference after the start. This is just over 2 seconds in, but is SPEED DEPENDANT, so some variation is likely.

Lap, Time to First Marker
1 2.31
2 2.52
3 2.28
4 2.64
5 2.38

Given likely variations in speed and limited frame to frame resolution, I'd say there was likely very little deviation from a constant starting point.

Finish times gives a better opportunity as they are not dependant on speed but on on errors introduced by a start point offset. For this test I logged the difference between finish time and time passing a visual marker near the finish (I have no way to know exactly where the HLT GPS finish is on the video):

Lap,Finish Time,Finish Marker,Delta
1 69.30 69.27 -0.03
2 66.45 66.39 -0.06
3 65.85 65.81 -0.04
4 65.04 65.00 -0.04
5 64.28 64.24 -0.04

So if we assume that the finish marker is 0.04s at speed away from the finish trigger there is only +/-0.02s variation and that includes variation introduced at the start trigger. I'd say that is impressive!

I hope to have official times soon and that will also indeed be interesting!

db

Slakker

That's closer to what I would expect. As long as the triggers are operating properly and you have a good signal on the Dual, the timing calculations are pretty simple and unlikely to have errors.

dbbarron

I now have my RAW times and below is the comparison to HLT times.

Unfortunately, I now realize that in fine tuning my start and finish triggers on the HLT map view, I likely moved them a bit so they were not directly aligned with the lights. To the extent my start or finish speed was different run to run between the trigger points and start/finish lights that would show itself as a varying offset from the official time. I did some rough calcs and this could easily be several tenths depending on relative speed at the start and finish. Moreover, I did note difference of up to 5-10 mph in start/finish speed as shown below. For similar start/finish speeds, the offset is similar. Maybe a tenth or two apart. Thus, it would appear HLT may align well with official times and should be quite usable for run to run analysis in AX. Net time I take more care in setting trigger points locations.

Lap Official Time HLT time Differential Start Speed Finish Speed
1 70.338 69.300 1.038 29.3 56.7
2 67.453 66.450 1.003 30.3 57.8
3 66.663 65.850 0.813 31.6 58.0
4 66.605 65.040 0.565 39.3 63.8
5 65.010 64.280 0.730 41.2 63.6

db

dbbarron

Thank you for the kind offer. Now that I have figured out the accuracy, I think I have sufficient GPS fixes and accuracy for my purposes. It would be interesting to compare twice as many fixes with half the accuracy (again, on paper); but I don't think it will be a material difference.

I have attached a screen capture from this weekend comparing a good line v. a bad line around a fairly tight corner. Plenty of GPS fixes to clearly see the difference in line and why. FYI, this represented 1.5s difference between the two runs (the bad line was actually faster around the remainder of the course - just overdrive/took a bad line through that particular corner). Great learning tool for me; I needed to enter wider and slower in that situation. I took closer to the bad line in most of my runs.

db

ProCoach

I know you would see a benefit in the CEP with the VBOX Sport.

dbbarron

"CEP"? (I tried searching the forum) - oops just found it - Circle of Error for the GPS position accuracy.

VBOX is spec'd at +/- 3m 95% CEP.
XGPS160 is spec'd at +/- 2.5m CEP (percentage unspecified).

I'm just wondering at the level/limitations of HLT or other apps whether this is a material difference.
A VBOX sport would be several hundred dollars towards an overall better system (wish I could find one that gave me both the instant review with video of HLT for AX and laptop level analytics (CircuitTools) for DEs).

db